Thermoplastic polyesters are very important polymer materials, produced commercially in large quantities. Linear thermoplastic polyesters such as poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) are used in a variety of forms. For example, they may be used in the form of synthetic fibers, which exhibit good resistance to most mineral acids, and exhibit excellent resistance to cleaning solvents and surfactants. Thermoplastic polyesters are also used to a great extent as molding materials. Such materials are often highly crystalline, and are characterized by many desirable properties, such as hardness, strength, toughness, good chemical resistance, and low moisture absorption.
On a commercial scale, thermoplastic polyesters are usually produced in a two-stage polymerization process. The first stage is often referred to as "pre-condensation" or "ester interchange". It involves oligomer formation by ester interchange of dimethyl terephthalate (DMT) with a dihydroxy compound, such as a glycol. Alternatively, the first stage can involve esterification of an aromatic acid such as terephthalic acid with a dihydroxy compound.
The second stage is often referred to as "polycondensation". In this stage, the oligomer formed in the first stage is polymerized at elevated temperatures, as excess glycol is removed. The overall reaction can be carried out as a batch process, but is typically a continuous operation, using two or more reactors.
Titanium-based compounds such as tetra-n-butyl titanate or tetraisopropyl titanate are often used to catalyze the reactions described above. As described in U.S. Pat. No. 5,519,108, for example, PBT can be prepared by reacting DMT and 1,4-butanediol at about 220.degree. C. to about 260.degree. C., using the titanium catalyst to initiate and accelerate the ester interchange reaction.
The presence of an effective catalyst is critical to the manufacturing process, especially in large scale operations where high yields and rapid processing times are required for economic production. The organotitanate compounds are effective to some degree in the production of polyesters like PBT. However, the rate of transesterification in the presence of such catalysts is relatively slow. Thus, long residence times are sometimes required to produce high molecular weight resins on an industrial scale. Furthermore, higher reaction temperatures may be required, thereby producing undesirable carboxylic acid end groups on the polymer product being formed. The presence of these types of end groups wastes the diol-type component, e.g., the butanediol, via conversion to unrecoverable side products such as tetrahydrofuran (THF).
The ultimate effects of using a relatively slow catalyst in polyester production are lower reactor capacity, lower productivity, and higher production costs. Thus, any improvement in the catalyst should result in higher reactivity which will, in turn, eliminate or minimize the undesirable effects described above.
Alternative catalyst systems for the production of polyesters like PBT have been described in the prior art. For example, U.S. Pat. No. 5,519,108 (Yuo et al) describes the use of a titanium compound as a primary catalyst, along with a cocatalyst system which includes a combination of at least one compound based on Zn, Co, Mn, Mg, Ca, or Pb, with various phosphite- or phosphate-based compounds. Another example is provided in an article entitled PET Synthesis in the Presence of Lanthanide Catalysts (J. Appl. Poly. Sci., 1995, Vol. 58, pp. 771-777). In that article, V. Ignatov et al. describe the use of various lanthanide catalysts in the production of PET.
Despite some of the advances described above, there continues to be a need for new catalysts or catalyst systems for preparing thermoplastic polyesters like PET and PBT. The new catalysts should provide high reactivity during preparation of the polymer products. Moreover, use of the catalysts should not lead to an excessive level of undesirable reaction byproducts. The new catalysts should also be relatively economical to use. Finally, polyester products formed by polymerization processes employing the new catalysts should have substantially the same property profiles as products based on the use of conventional catalysts.